[AggieDad]

After years of nodding sagely with feigned understanding, I finally admitted to myself my ignorance and yielded to my curiosity and the better angels of my education. I went off to find out just what is meant by the term “focusing beyond infinity.”

By way of background, I am a mathematics teacher. So you may be able to understand my consternation when I would read about a lens’ ability (or inability) to focus past infinity. To a math guy, this just isn’t going to happen. Infinity is something that is boundless, or, mathematically, it is something that is larger than any real number.

In other words, by definition, there is nothing beyond infinity.

So I have read a number of explanations (including this one on the Forum and this very fascinating article on the B&H website), and I think I kinda, sorta, maybe understand the concept. What I don’t understand is the poorly chosen phrase to describe what is being done.

Perhaps the better phrase would be “past the ∞ mark.” But then as Timothy H. asked in a response to the B&H article, “...exactly where on the infinity mark is infinity?”

Isn’t it interesting how questions beget other questions…

[bertwert]

In my limited understanding of optics, infinity is not so much an unbounded value, but rather the distance where a light source is far enough away that the rays are effectively coming in parallel to each other. And that the phrase "focusing at infinity" means the focus arrangement that parallel light rays will be perfectly focused on the sensor.

As far as focusing past infinity, you're moving the focal arrangement of the lens past the point where optimal focus is achieved with parallel light rays, that is overfocused in a sense. And by the way optics work, nothing will ever be in focus once the optics are moved past 'infinity' focus.

At least that's my interpretation, and it could be completely wrong

[Bob 256]

My eyes, when I was younger, could focus very well on objects "approaching infinity" - stars and the like as well as very distant terrestrial objects. As I have grown older, my eyeballs have shrunken which has moved the retina towards my eye lens, and beyond the infinity focus point (on the wrong side). So, if you choose a non-mathematical approach (maybe an engineering approach) to this issue, you might say my eyes now focus "beyond infinity". I can prove that, because if I place a positive lens of slight strength in front of my eyes, it effectively shortens the focal length of my eyes and returns them to an infinity focus, the equivalent of subtracting something from infinity and getting a finite number (pretty neat math,eh)

I think the bottom line here is that lens positioning can be arranged to go beyond the mechanical point where focus on objects at (near) infinity is optimum, and thus the (near) infinity object goes out of focus. Obviously, a lens is not focusing on an object which lies "beyond infinity" anymore than the appropriateness of Buzz Lightyear's comment of "infinity and beyond", but it certainly can mechanically move beyond the infinity setting.

[sergysergy]

In other words, by definition, there is nothing beyond infinity.

As you mention towards the end of your post I think in most cases people talk about the lens going beyond the infinity mark.

[mee]

Focusing beyond infinity -- When your lens goes plaid.

[othar]

Perhaps the better phrase would be “past the ∞ mark.” But then as Timothy H. asked in a response to the B&H article, “...exactly where on the infinity mark is infinity?”

Isn’t it interesting how questions beget other questions…

While I don't think this was a serious suggestion this wouldn't really change anything in my opinion.
As ∞ is the mathematical character for infinity I don't see the point of writing “past the ∞ mark", because when you read this aloud you would still say infinity ("horizontal eight mark" doesn't sound good ).

Infinity is a concept hard to wrap your head around and as the stars are so far away noone would survive a trip to the next one (aside from our sun, a journey we wouldn't survive for obvious reasons too) they could basically be infinitely far away without making much difference (from the perspective of one's individual life span)

[CarlJF]

It just means the focus ring can be turned passed the infinity point. This extra play is needed for the autofocus system to work correctly and compensate for thermal expansion or contraction. Obviously, nothing is in focus beyond this point…

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Last edited by CarlJF; 05-07-2021 at 08:56 AM.

[EssJayEff]

In other words, by definition, there is nothing beyond infinity.

I wonder what happens to infinity inside a worm hole? Perhaps one could photograph beyond infinity there, but I guess we can't photograph inside one . . . unless you have a very tiny camera and look toward the soil rather than the sky!

[AstroDave]

Consider the plane of focus in the camera (where the film or sensor is). If a lens is correctly focused for infinity, then light from e.g. a star will be in focus on the film. If you move the lens farther from that position (i.e. moving the focus ring to indicate "beyond infinity"), the plane of focus moves to be in front of the film/sensor. The lens is still in some sense focused on infinity, but the focal point no longer coincides with the sensor. Alternatively, think of it as moving the lens assembly closer to the object, without a comparable moving of the sensor.

And, of course, to some extent (as indicated by various forum questions/postings: Testing Pentax 28mm infinity focus. - PentaxForums.com) the ring with the focus distances written on it can be positioned (on purpose, by mistake, slippage due to aging, ...) so that the distances have little bearing on reality!

Take a look at the lensmaker's equation about a quarter of the way into the wikipedia article (Lens - Wikipedia). As a math teacher, you should dig this stuff!

[stevebrot]

In my limited understanding of optics, infinity is not so much an unbounded value, but rather the distance where a light source is far enough away that the rays are effectively coming in parallel to each other. And that the phrase "focusing at infinity" means the focus arrangement that parallel light rays will be perfectly focused on the sensor.

As far as focusing past infinity, you're moving the focal arrangement of the lens past the point where optimal focus is achieved with parallel light rays, that is overfocused in a sense. And by the way optics work, nothing will ever be in focus once the optics are moved past 'infinity' focus.

At least that's my interpretation, and it could be completely wrong

If you are wrong, then I am too.

At issue is a misuse of the term "infinity". By definition, one can't get there. In practical terms, it means the distance where going further is beyond the ability of optical system to resolve an object of a given size any further out. I have several legacy wide angle lenses whose infinity stops are set comically close due to neither the viewfinder focus aids nor the image at final enlargement sizes, at the time the lenses were made, being capable resolving further distance. There are also the limits imposed by intervening kilometers of atmosphere.

I am reminded that the same is true, though to a lesser extent, with current lens offerings despite improvements and optics and high resolution sensors. A few weeks ago I attempted to dial in "infinity", or a semblance thereof, on my Samyang 14/2.8. With that lens on my K-3 infinity appears to be about 50 meters away in regards to texture and about 200 meters for anything other than large building and Mount Hood.


Steve

[MossyRocks]

I am a mathematics teacher. So you may be able to understand my consternation when I would read about a lens’ ability (or inability) to focus past infinity.

Maybe by focusing beyond infinity you now have focused to an uncountable infinity, thus making nothing in the image in focus because of all those overlapping infinities.

Now that I have got that dumbness out of the way it is a rather odd concept, focusing beyond infinity, and I have never thought of it until now. The explanations here should help others understand it though.

This also got me thinking about the related concept of depth of field and the advise that I give to people who are starting out in astrophotography: From your len's perspective (hand wavy to cover most lenses most people might have) everything from about the distance that airplanes cruse at to the farthest galaxy is at infinity so you don't need to stop down to increase the depth of field for stuff in space.

[stevebrot]

Maybe by focusing beyond infinity you now have focused to an uncountable infinity, thus making nothing in the image in focus because of all those overlapping infinities.

That is why I used the term "undefined" in my first edit, but removed that sentence.


Steve

[DWS1]

After years of nodding sagely with feigned understanding, I finally admitted to myself my ignorance and yielded to my curiosity and the better angels of my education. I went off to find out just what is meant by the term “focusing beyond infinity.”

By way of background, I am a mathematics teacher. So you may be able to understand my consternation when I would read about a lens’ ability (or inability) to focus past infinity. To a math guy, this just isn’t going to happen. Infinity is something that is boundless, or, mathematically, it is something that is larger than any real number.

In other words, by definition, there is nothing beyond infinity.

So I have read a number of explanations (including this one on the Forum and this very fascinating article on the B&H website), and I think I kinda, sorta, maybe understand the concept. What I don’t understand is the poorly chosen phrase to describe what is being done.

Perhaps the better phrase would be “past the ∞ mark.” But then as Timothy H. asked in a response to the B&H article, “...exactly where on the infinity mark is infinity?”

Isn’t it interesting how questions beget other questions…

I agree with you. The "focus past infinity" phrase has always bothered me too, and I'm not even a math teacher. Just someone who understands basic science.

[mee]

Focusing beyond infinity -- When your lens goes plaid.

Aww no one enjoyed my space balls joke. Ah well.

[pentaxus]

With changes in temperature, lens elements in lenses expand and contract. Lenses focus past infinity to be sure that they can focus to infinity when the weather is cold. A friend had a Takumar 400mm 5.6 that focused past infinity in warm weather but when the temp got below -10C the lens elements got enough thinner that it couldn't focus at infinity.